Deformable-mirror holder

ABSTRACT

This invention relates to a deformable-mirror holder for holding a mirror in a desired position, to within accepted tolerances, even whilst the mirror is deforming or in a deformed state. In particular, this invention relates to a holder for a bimorph mirror. A deformable-mirror holder is provided comprising a body with a central aperture for receiving a deformable mirror, the central aperture being defined by a plurality of flexible beams, with each flexible beam having an end shaped to provide a supporting surface and a flexible portion that connects the beam&#39;s end to the holder&#39;s body.

This application is the U.S. national phase of international applicationPCT/GB2003/005547 filed 18 Dec. 2003 which designated the U.S. andclaims benefit of GB 0230038.2, filed 23 Dec. 2002, and GB 0309979.3filed 30 Apr. 2003, the entire contents of each of which are herebyincorporated by reference.

This invention relates to a deformable-mirror holder for holding amirror in a desired position, to within accepted tolerances, even whilstthe mirror is deforming or in a deformed state. In particular, thisinvention relates to a holder for a bimorph mirror.

Deformable mirrors are often used in the field of adaptive optics. Forexample, phase distortions in a signal may be sensed by a wavefrontsensor and these distortions may be corrected for by an adaptive mirror.Such adaptive mirrors may be employed in numerous fields, including:

-   -   imaging, for example adaptive mirrors are used in astronomy to        improve the resolution of earth-based telescopes that are        otherwise affected by atmospheric distortions;    -   laser sensing, where the amount of laser light that can be        delivered onto a target is significantly increased by using an        adaptive mirror to correct for atmospheric distortions—this        enables either better information to be obtained or objects to        be identified at a greater range; and    -   laser generation, where an adaptive mirror can be used        intracavity within a high power laser to counter the thermal        blooming that can be otherwise induced by the high concentration        of laser light inside the cavity.

Bimorph deformable mirrors have been proposed as low cost adaptivemirrors. The two main operational parameters of a bimorph mirror are itsbandwidth and its stroke. Bandwidth determines how quickly the mirrorcan be deformed and hence, for example, how quickly the mirror canrespond to the variations in atmospheric turbulence. Stroke correspondsto the maximum displacement of the mirror when deformed and thisdetermines, for example, the level of turbulence that can be corrected.Ideally, both bandwidth and stroke would be maximised. However,conventional designs mean that there is a reciprocal relationshipbetween these two parameters, and one parameter can only be improved atthe expense of the other. Therefore, to date, designers have alwayslooked for ways to improve either the resonant frequency or the strokeindependently from each other.

Conventionally, deformable mirrors are supported rigidly around theiredge, for example an annular ring overlapping the periphery of themirror is used to hold the mirror firmly in position. Such anarrangement benefits from being simple yet rugged. However, it has aninherent disadvantage in that it creates a dead space around themirror's edge. This corresponds both to the area of the mirror heldfirmly under the annular ring and also to the adjacent area. This isbecause the useable area of the deformable mirror (the active area) mustbend to adopt a desired profile, for example either a concave or aconvex shape. The annular area between the active area and the annularring must bend in the opposite sense and so forms an area of inflexionthat has undesirable optical properties. Hence the active area occupiesonly a central portion of the whole mirror. This is illustrated in FIGS.1 to 3, albeit with the deformation of the mirror exaggerated for thesake of clarity.

One way of alleviating this problem is to clamp the mirror at only threepositions, such that the mirror edge can twist. However, thisarrangement is to the detriment of ruggedness and the twistingintroduces unwanted distortions leading to an optically inferiorperformance.

Against this background, the present invention resides in adeformable-mirror holder comprising a body with a central aperture forreceiving a deformable mirror, the central aperture being defined by aplurality of flexible beams, with each flexible beam having an endshaped to provide a supporting surface and a flexible portion thatconnects the beam's end to the holder's body. The supporting surface isprovided for supporting a peripheral edge of the mirror.

Accordingly, we have devised a deformable-mirror holder that simplysupports the mirror uniformly around the edge and where the beamsdeflect with the mirror as it deforms, thereby allowing the deformablemirror to move like a simply-supported diaphragm. In this way, thedeformable mirror is supported such that the ratio of the total diameterto active diameter is minimised. In addition, the above holder allowsthe mirror to be as small as possible.

Optionally, the ends of the flexible beams are co-joined to form aunitary structure shaped to provide a supporting surface. Such astructure would support the mirror around the entirety of its peripheraledge.

Preferably, the beams' ends lie in the plane of the mount's body suchthat, in use, the mirror is positioned within the mount's body. Thissimple arrangement ensures that the holder's body affords the mirror anelement of protection. Alternatively, the beam's ends may lie outside ofthe plane of the body such that the mirror is held clear of the holder'sbody.

Optionally, at least one beam is generally L-shaped such that one leg ofthe L-shape provides the flexible portion and the other leg of theL-shape provides the supporting surface of the beam's end. In acurrently preferred embodiment, the internal corner of the L-shaped beamhas a shoulder that extends part of the way along both legs of theL-shape. This adds rigidity to the part of the beam that supports themirror in use and a flexible neck is created that allows the supportingend to bend with the mirror.

Preferably, the plurality of flexible beams are arranged around theentire aperture. Alternatively, a small number of beams may be providedaround the aperture, for example three or four equispaced around theaperture. Where the beams are arranged around the entire aperture, thewidth of the beams may optionally be larger than the separation betweenbeams. Preferably, the width of the beams is greater than four times theseparation between beams. This arrangement means that the holder isflexible in the radial direction, but exhibits stiffness in all otherdirections.

Optionally, the flexures are shaped to provide vertical movement at themirror's edge. Each of the flexures could have a bar shape forsupporting a continuous ring, which in turn could be used to support themirror for example.

Optionally, wire bonding techniques can be effectively applied in thepresent invention where a large number of connections to piezoelectric(PZT) elements on a mirror are required to be made.

The present invention also extends to a deformable mirror and adeformable-mirror holder as described above.

In order that the invention can be more readily understood, referencewill now be made, by way of example only, to the accompanying drawingsin which:

FIG. 1 is a plan view of a deformable mirror and a mount according tothe prior art;

FIG. 2 is a cross-section through line II-II of FIG. 1 showing themirror in a relaxed state;

FIG. 3 corresponds to FIG. 2 but with the mirror in a state ofexaggerated deformation thereby illustrating the problem of dead spaceand the relatively small active area;

FIG. 4 is a plan view of a deformable mirror and a mount according to afirst embodiment the present invention;

FIG. 5 is a cross-section through line V-V of FIG. 4 showing the mirrorin a relaxed state;

FIG. 6 corresponds to FIG. 2 but with the mirror in a state ofdeformation;

FIG. 7 is a detail from FIG. 5;

FIG. 8 is a perspective view of part of the mount of FIG. 5;

FIG. 9 corresponds to FIG. 7 but for a second embodiment of the presentinvention;

FIG. 10 corresponds to FIG. 7 but for a third embodiment of the presentinvention;

FIG. 11 is a cross-sectional view of a fourth embodiment of the presentinvention;

FIG. 12 is a perspective view of part of the mount of FIG. 11, with themirror removed;

FIG. 13 is a further perspective view of a part of the mount of FIG. 11,with the mirror in place;

FIG. 14 is a view of a bar flexure design embodying the presentinvention from above (a) and below (b);

FIG. 15 is a cross-sectional view of the embodiment of FIG. 14;

FIG. 16 is a view of the mirror mount using the bar flexure design ofFIG. 14 (without the mirror in place); and

FIG. 17 shows one way of effectively applying wire bonding techniquesfor the purpose of making connections to the piezoelectric (PZT)elements in an embodiment of the present invention.

A deformable bimorph mirror 10 and its mount 12 according to the priorart are shown in FIGS. 1 to 3. As can be seen, the mirror 10 isdisc-shaped and is supported by the mount body 12. The mirror 10 is heldfirmly in position by an annular ring 14 that urges the mirror 10against the mount body 12 via four screws 16. FIG. 3 shows the mirror 10in a deformed state such that, in this example, it adopts a convexprofile for light approaching from above. Ideally, the desired convexprofile should extend across all the mirror 10, such that all the mirror10 is available for reflecting light in the desired manner.

However, the peripheral edge 18 of the mirror 10 is held firmly betweenthe annular ring 14 and the mount body 12 and so cannot bend. Moreoverthere is a region 20 of the mirror 10 that adopts an area of inflexionto bridge the peripheral edge 18 of the mirror 10 and the convex portion22 at the centre of the mirror 10. It is of course this convex-shapedpart 22 of the mirror 10 that forms the active (i.e. useable) part 22 ofthe mirror 10. This active part 22 of the mirror 10 is shown in FIG. 3.

A deformable bimorph mirror 50 and a mount 52 according to a firstembodiment of the present invention are shown in FIGS. 4 to 8. The mount52 in this case is simpler when compared to the prior art in that themount 52 is a unitary structure made from stainless steel. The mount 52comprises a round body 54 that defines a central circular aperture 56.The aperture 56 is shaped and sized to receive the disc-shapeddeformable bimorph mirror 50 therein. Hence, the mirror 50 is held in aprotected position within the mount 52.

Whilst the outer edges of the mount's body 54 are regular, the internaledges 58 are stepped to form a series of three interconnected andconcentric circular apertures 56 a-c that increase in size from top tobottom. The stepped inner profile 58 of the mount 52 produces a seriesof three shoulders 60 a-c. Twenty generally L-shaped flexible beams 62extend downwardly in cantilever fashion from the topmost 60 a of theseshoulders 60 a-c. The twenty beams 62 are of identical size and shapeand are equispaced around the circular topmost shoulder 60 a. The beams62 are L-shaped such that they extend downwardly from the topmostshoulder 60 a before turning through 900 to extend inwardly towards thecentre of the middle aperture 56 b. Rather than having a pure L-shape, asquare-shaped support shoulder 64 extends from the internal corner ofeach beam 62 as best seen in FIG. 7. The support shoulder 64 onlyextends partially up the height of the upright portion 66 of the beam62, thereby leaving a narrow neck 68 in the portion of the beam 62 thatbridges the topmost shoulder 60 a of the mount body 54 and the supportshoulder 64 of the beam 62.

It is this neck 68 that gives the beam 62 its flexibility, i.e. thisneck 68 can be deformed to allow the beam 62 to deflect and bend. Thelength and thickness of the neck 68 of the beams 62 are chosen toachieve the desired flexing properties. FIG. 8 shows four of the beams62 in perspective and indicates the width W of the beams 62 relative totheir separation. It is the relative width of the beams 62 that givesthe required degree of stiffness in the plane of the mirror 10.

The inwardly-extending portion 70 of the beam 62 extends beyond thesupport shoulder 64 to provide an upwardly-facing support surface 72 forreceiving the mirror 50. The mount 52 and the beams 62 are sized suchthat the mirror 10 may be received within the beams 62 to be supportedfrom below by the support surfaces 72 and so that the mirror's edge 74fits snugly against the upright face 76 of the support shoulders 64.Hence, the mirror 50 is held firmly in place.

The mirror 50 is best seen in FIG. 5. The mirror 50 comprises a coppersubstrate 78 whose outer face provides a reflecting surface by virtue ofa series of thin dielectric coatings provided on the outer surface (notshown). An active piezoelectric element 80 is bonded to thenon-reflective side copper substrate 78 using epoxy resin 82. It isimportant to ensure that a thin, uniform layer of the epoxy resin 82 isused to bond the piezoelectric element 80 to the substrate 78 in orderto ensure maximum (and uniform) coupling efficiency of stress from thepiezoelectric element 80 to the substrate 78. In this embodiment, thisis achieved by mixing a small amount of spacers with the glue (these canbe for example, silica/glass/polymerics/spheres or rods). This enables aknown, uniform thickness of epoxy resin/glue to be formed. An array offorty-five electrodes 84 are used to activate the piezoelectric element80. Applying a potential to the electrodes 84 causes the piezoelectricelement 80 to deform so that, in turn, the copper substrate 78 deforms,as shown in FIG. 6. This creates a convex-shaped mirror 50.

As the mirror 50 deforms, it remains firmly held in place against thesupport surface 72 and support shoulder 64 because the beam 62 deflectswith the mirror 10 by flexing about its neck 68, as shown in FIG. 6.Moreover, the beams 72 offer minimal resistance to the mirror 50 as itsperipheral edge 74 rotates towards the mirror axis. This is because theyhave minimal stiffness radially and so require little force to deformradially towards the mirror centre. The mass and stiffness of the beams62 are very small in comparison to that of the mirror 50 and thereforethe beams 62 have minimal impact upon the mirror 50 deformation. Inaddition, the relatively large width W of the beams 62 providesstiffness in all directions in the plane of the mirror and torsionallyabout the mirror axis. The short length of the beams 62 providesstiffness in the axial direction.

FIG. 6 shows that convex deformation of the mirror 50 extends to thevery edge 74 of the mirror 50 and hence eliminates virtually all deadspace from the mirror 50. Hence, the active area of the mirror 50 coversvirtually the whole of the mirror 50. This is highly beneficial becausea mirror mount 52 that prevents rotation of the mirror's peripheral edge74 would need to be twice the diameter to obtain a similar convex activearea and would have a first mode resonant frequency of half that of thesimply supported mirror 50 of the present invention. Thus, the presentinvention allows for a mirror 50 of much smaller size to be used toobtain the same stroke/bandwidth product.

The person skilled in the art will appreciate that modifications can bemade to the embodiments described hereinabove without departing from thescope of the invention.

Details of the mirror 50 and how it is arranged to deform are given asuseful background in which to set the context of the present invention,but are not essential to the invention. Other mirror configurations canbe equally well accommodated by the present invention.

Whilst the above embodiment uses L-shaped beams 62, strict compliancewith this shape is not necessary. For example, the support shoulders 64may be omitted and the peripheral edge of the mirror 74 may abut againstthe upright face of the beam 62. This arrangement would lead to a longerneck 68 that could flex along its entire height. In addition, the beam62 could be J-shaped rather than being L-shaped. This may beadvantageous where the mirror 50 has rounded edges rather than squareedges. In fact, the beam 62 may be shaped to conform to any profile themirror 10 may have, e.g. to conform to chamfered edges.

Furthermore, the beams 62 need not necessarily extend downwardly fromthe mount body 54 to house the mirror 50 within the mount body 54. Analternative arrangement is shown in FIG. 9, that broadly corresponds tothe view shown in FIG. 7 and so like reference numerals have been usedfor like parts but with the addition of a prime. In this embodiment, theflexible neck 68′ is L-shaped such that, in addition to the flexibleupright portion that allows deflection as the mirror 50 deforms, thereis a horizontal portion 88′ that connects the upright portion to themount body 54. The horizontal portion 88′ of the beam 62′ allowsvertical movement of the edges of the mirror 50, as indicated by thearrows in FIG. 9. This is beneficial because the mirror 50 may bedeformed to adopt shapes that require relative movement around the edge74 of the minor 50, e.g. to adopt radially-extending ridges and troughsthereby creating an undulating mirror edge 74.

A further alternative arrangement of the beams 62 is shown in FIG. 10where beams 62″ extend upwardly from the mount body 54″ (like referencenumerals are used for like parts, the double prime denoting the partsthat belong to the embodiment of FIG. 10). Most importantly they retainthe flexible neck 68″ that allows the beam 62″ to bend with the mirror(not shown) as it adopts a convex shape.

A yet further embodiment is shown in FIGS. 11 to 13. Again, likereference numerals are used for like parts, the triple prime denotingthe parts that belong to the embodiment of FIGS. 11 to 13. In thisembodiment, the mirror 50′″ is supported at the top of the mount 52′″.The mount 52′″ has an outer wall 90′″ extending from the outer edge ofits top surface. Twenty flexible beams 62′″ extend from the inner edge60 a′″ of the mount 52′″. The flexible beams 62′″ comprise an L-shapedflexible neck 68′″ that extends from the mount 52″″ first upwardly as anupright portion 86′″ before turning through 90° to extend inwardly as ahorizontal portion 88′″. The horizontal portion 88′″ of each of theflexible beams 62′″ meets a unitary L-shaped annular ring 92′″ that isshaped and sized to receive the mirror 50′″. The L-shape of the ring92′″ is such that it supports the mirror 50′″ from the side and frombelow.

The advantage of this arrangement is that the shape of the flexiblebeams 62′″ allows vertical movement of the mirror's edge. This providesadditional enhancement by further minimising the ratio of the totaldiameter of the mirror 50′″ to the active diameter. This reduces theoverall mirror diameter required to achieve a given stroke for a setapplied voltage and bandwidth by virtually eliminating any dead spacefrom the outside of the mirror 50′″.

A yet further embodiment of the invention is shown in FIGS. 14 to 16.FIG. 14 provides a view of a bar flexure design from above (a) and below(b). FIG. 15 corresponds to a cross-sectional view of FIG. 14 throughone of the bar flexures 150. FIG. 16 is a view of the whole mirror mount(without the mirror) using the bar flexure design of FIG. 14. In thisembodiment, the flexure is a bar 150 supporting a continuous ring, whichin turn is used to support the mirror, but in this case fewer flexuresare required. Conveniently, as shown, a hole is machined down the middleof the bar flexure in order to reduce the rigidity of the flexure.

The advantage of the arrangement shown in FIGS. 14 to 16 is that the barshape of the flexures allows vertical movement at the mirror's edge.This arrangement bears the additional advantage of being astraightforward design, making the mirror holder easy to machine.

It is also to be appreciated that standard methods for makingconnections to the piezoelectric (PZT) elements on a bimorph mirror areto either solder the wires on, or to use a conducting epoxy. Both ofthese methods are effective when the number of electrodes is relativelysmall e.g. up to 40. However, as the number of electrodes go up, makingthe connections becomes progressively more difficult:

-   -   soldering onto the PZT element needs to be done quickly in order        to ensure that the material does not de-pole slightly in the        process. When 40 solder joints are required, this becomes more        difficult. If all the electrodes are soldered at the same time,        the risk of the material de-poling becomes that much greater.        This can be avoided by letting the material cool down before        starting the next joint, but this is time consuming.    -   Using conductive epoxies is not that easy either. Care needs to        be taken to ensure that not too much epoxy is used each time in        order to avoid adding extra load onto the system. Also, the        wires typically need supporting while the epoxy sets.        Wire bonding is seen by the inventors as a possible alternative        to the above described techniques, but care must be taken in        order to ensure that the wires do not touch. The inventors have        recognised that careful application of the wire bonding        technique bears the following definite advantages:    -   Wire bonding can be automated, making the process quick and        consistent with commercial assembly.    -   Because the footprint of a wire bond is so small, the process        does not add any significant load to the disc, even when a large        number of connections (e.g. >100) are required.    -   The expected current to each element is relatively small (6 mA)        and is consistent with wire bonding techniques.    -   The bend that is normally put into the bond wire during the        bonding process ensures that the small amount of movement that        the bond will be subjected to will not weaken the bond or the        wire.

Having regard to the foregoing, wire bonding can be provided and FIG. 17shows one way of accomplishing this. Again, like reference numerals areused for like parts, the four primes denoting the parts that belong tothe embodiment of FIG. 17. As shown in FIG. 17, a hole 101″″ is punchedthrough the PCB 102″″ above each piezoelectric (PZT) electrode 84″″ inorder to enable access. This hole could typically be -2mm in diameterand still provide access. As shown, the PCB sits very close to the PZTdisc 80″″, but must be at least twice as far as the movement expectedfrom the mirror. The wire bond 103″″ makes the connection between thePZT electrode 84 and the PCB track 104″″. A solder pin 105″″ can be usedfor soldering to an external cable. The holder for the PCB 106″″ isattached to the mirror base 54″″.

It is envisaged that this wire bond technique can be suitably used withother types of mounting arrangement if desired (for example, where themirror is fixed rigidly).

As will be appreciated by the skilled person, other arrangements of theflexures/beams are possible. For example, the flexible beams 62 couldextend inwardly to meet a supporting end of the beam 62. Essentially,any arrangement could be used where the supporting end of the beam 62 isconnected to the mount body 54 by a flexible neck 68 that allows thesupporting end to bend as the mirror 10 deforms.

Whilst the mount 52 of the above embodiments is made from stainlesssteel, many other materials such as other metals, plastics, glasses orceramics could be used instead.

The present invention is perfectly well suited for use in supportingboth uncooled and cooled bimorph mirrors. For example, the bimorphmirror may be water-cooled in order to dissipate heat absorbed fromincident radiation.

1. A deformable mirror apparatus comprising: a deformable mirror, amounting body having an aperture in which the deformable mirror isreceived, and flexible structure extending across the aperture to themirror, and a deformation device for controllably deforming the mirror,the deformation device acting on the mirror other than through theflexible structure comprising a plurality of flexible beams arrangedaround the entire periphery of the aperture, the flexible structurehaving an end shaped to provide a supporting surface supporting themirror, and a flexible portion linking the supporting surface to thebody and permitting movement of an edge of the mirror relative to thebody when the mirror is deformed by the deformation device, wherein atleast one beam is generally L-shaped such that one leg of the L-shapeprovides the flexible portion and the other leg of the L-shape providesthe supporting surface of the end of the beam, wherein the peripheraledge of the mirror is supported from below by one leg of the L-shapedbeam and is supported from the side by the other leg of the L-shapedbeam.
 2. A deformable-mirror holder comprising: a deformable-mirrorholder comprising a rigid body with a central aperture for receiving adeformable mirror, the mirror having a deformation device for deformingthe mirror attached thereto, and a plurality of flexible beams aroundthe entire periphery of said central aperture for supporting said mirrorat an edge thereof, each flexible beam comprising a support for saidmirror, said support permitting movement of said mirror edge when saidmirror is deformed by said deformation device, said support including anend shaped to provide a supporting surface for supporting saiddeformable mirror and a flexible portion that links said end of the beamto said body of the holder, wherein at least one beam is generallyL-shaped such that one leg of the L-shape provides the flexible portionand the other leg of the L-shape provides the supporting surface of theend of the beam, wherein the internal corner of the L-shaped beam has ashoulder that extends part of the way along both legs of the L-shape. 3.A deformable-mirror holder as in claim 2, wherein the ends of theflexible beams are co-joined to form a unitary structure shaped toprovide said supporting surface.
 4. A deformable-mirror holder as inclaim 2, wherein the ends of the beams lie in the plane of the body ofthe holder such that, in use, the mirror is received within the body ofthe holder.
 5. A deformable-mirror holder as in claim 2, wherein thewidth of the beams is larger than a separation between beams.
 6. Adeformable-mirror holder as in claim 5, wherein the width of the beamsis greater than four times the separation between beams.
 7. A deformablemirror holder as in claim 2, wherein the body is a unitary structure. 8.Deformable mirror apparatus comprising: a deformable mirror, a mountingbody having an aperture in which the deformable mirror is received, andflexible structure extending across the aperture to the mirror, and adeformation device for controllably deforming the mirror, thedeformation device acting on the mirror other than through the flexiblestructure comprising a plurality of flexible beams arranged around theentire periphery of the aperture, the flexible structure having an endshaped to provide a supporting surface supporting the mirror, and aflexible portion linking the supporting surface to the body andpermitting movement of an edge of the mirror relative to the body whenthe mirror is deformed by the deformation device, wherein at least onebeam is generally L-shaped such that one leg of the L-shape provides theflexible portion and the other leg of the L-shape provides thesupporting surface of the end of the beam, wherein the internal cornerof the L-shaped beam has a shoulder that extends part of the way alongboth legs of the L-shape.
 9. A deformable mirror apparatus as in claim 8wherein the flexible portion permits rotation of the edge of the mirror.10. A deformable mirror apparatus as in claim 8 wherein the flexibleportion permits displacement of the edge of the mirror axially of themirror.
 11. A deformable mirror apparatus comprising: a deformablemirror, a mounting body having an aperture in which the deformablemirror is received, and flexible structure extending across the apertureto the mirror, and a deformation device for controllably deforming themirror, the deformation device acting on the mirror other than throughthe flexible structure, the flexible structure comprising a plurality offlexible beams, each beam having an end shaped to provide a supportingsurface supporting the mirror, and a flexible portion linking thesupporting surface to the body and permitting movement of an edge of themirror relative to the body when the mirror is deformed by thedeformation device, each beam being generally L-shaped such that one legof the L-shape provides the flexible portion and the other leg of theL-shape provides the supporting surface of the end of the beam, theperipheral edge of the mirror being supported from below by one leg ofthe L-shaped beam and being supported from the side by the other leg ofthe L-shaped beam.
 12. A deformable-mirror apparatus as in claim 11,wherein the ends of the flexible beams are co-joined to form a unitarystructure shaped to provide said supporting surface.
 13. Adeformable-mirror apparatus as in claim 11, wherein the ends of theflexible structure lie in the plane of the body such that the mirror isreceived within the body.
 14. A deformable-mirror apparatus as in claim11, wherein the width of the beams is larger than a separation betweenbeams.
 15. A deformable-mirror apparatus as in claim 14, wherein thewidth of the beams is greater than four times the separation betweenbeams.
 16. A deformable mirror apparatus comprising: a deformablemirror, a mounting body having an aperture in which the deformablemirror is received, and flexible structure extending across the apertureto the mirror, and a deformation device for controllably deforming themirror, the deformation device acting on the mirror other than throughthe flexible structure, the flexible structure comprising a plurality offlexible beams, each beam having an end shaped to provide a supportingsurface supporting the mirror, and a flexible portion linking thesupporting surface to the body and permitting movement of an edge of themirror relative to the body when the mirror is deformed by thedeformation device, each beam being generally L-shaped such that one legof the L-shape provides the flexible portion and the other leg of theL-shape provides the supporting surface of the end of the beam, andwherein the internal corner of the L-shaped beam has a shoulder thatextends part of the way along both legs of the L-shape.
 17. A deformablemirror apparatus as in claim 16, wherein the peripheral edge of themirror is supported from below by one leg of the L-shaped beam and issupported from the side by an inwardly-facing side of the shoulder.